This invention relates to rapid setting sprayable fireproofing compositions and to a method of spray applying the same to substrates which are to be fireproofed.
It is well known to spray apply hydraulic cement-based slurries to metal structural members in order to provide a heat resistant coating. A particularly successful type of product in this field is a gypsum-based formulation which contains, in addition to the gypsum binder, a lightweight inorganic aggregate, e.g., vermiculite, a fibrous substance, e.g. cellulose, and an air entraining agent. Such compositions are taught, for example, in U.S. Pat. Nos. 3,719,513 and 3,839,059. The slurries are generally prepared at ground level, pumped to the point of application, often exceeding 20 to 30 stories in high rise construction, and then applied through a spray nozzle.
In order to be suitable for such use, the slurries must possess a number of important properties. They must be able to hold a relatively large quantity of water such that they are sufficiently fluid to be pumped easily and to great heights. Yet they must retain a consistency sufficient to prevent segregation or settling of ingredients and provide an adequate "yield", or volume of applied fireproofing per weight of dry mix. The coating slurries, furthermore, must obviously adhere to steel surfaces, both in the slurried state and after setting. Also, the slurry must set without the undue expansion or shrinkage which could result in the formation of cracks that can deter from the insulative value of the coating.
A further factor which affects the formulation of the hydraulic cement-based compositions is set time. Fireproofing mixes, such as W. R. Grace & Co.'s MONOKOTE - brand materials, are transported to the building site as dry mixtures and an appropriate amount of water is added to form a slurry. The slurry is pumped from temporary holding equipment to the point of application. The preparation and application processes span many hours and thus the setting time of the mix is generally heavily retarded to provide an acceptable field pot life.
Although the addition of retarders provides a mix which remains pumpable over many hours, this long set time is undesirable once the composition has been applied to the structure. The unset composition has a lesser degree of adhesion to the metal substrate and thus a greater tendency to fall off the substrate. If applied under hot, dry conditions, the mix may dehydrate before setting and yield a less than optimum final product. In cold temperatures, the mix may freeze before setting, while the preferred sequence is setting before freezing. Where multiple layers of fireproofing are to be applied, the first layer must set at least partially prior to application of the next layer. The long set time thus requires the sprayman to move on to another portion of the structure, only to return several hours later for application of the next layer. This results in an inefficient use of manpower.